Paving roadways, driveways, parking lots, and the like with a bituminous aggregate mixture material is well known. Typically, a mixture of a suitable aggregate comprising stones, gravel, sand, and the like, is heated at an elevated temperature of about 270-370° F. and mixed with a similarly hot, bituminous binder such as an asphalt-based binder (e.g., asphalt or asphalt plus polymer) until the aggregate particles are coated with the binder. Paving mixes made in this temperature range are often referred to as a hot mix. The mixing typically occurs away from the paving site, and the mixture is then hauled to the site and supplied to a paving machine. The mixture of asphalt and aggregate applied by the paving machine to a surface is then usually roller compacted by additional equipment while still at an elevated temperature. The compacted aggregate and asphalt material eventually hardens upon cooling. Because of the large mass of material in paving a roadway or commercial parking lot, the cost of the thermal energy to achieve suitable mixing and paving is considerable. For common binders, the thermoviscosity characteristics of the binder affect the temperature needed to provide thorough coating of the aggregate and consideration of the ambient conditions suitable for paving. Consequently, numerous processes have been devised to optimize aggregate coating and pavement binding while minimizing the cost of materials and/or the process.
As alternatives to hot-mix processes, there are cold-mix processes, where the aggregate, cold and moist, is mixed with a hot or cold binder, which can be an emulsion of asphalt dispersed in water using a suitable surfactant or a mixture of asphalt and a suitable hydrocarbon solvent, such as naphtha, #1 oil, or # 2 oil, to name a few (generally referred to as a cutback asphalt). The emulsified asphalt particles coat and bind with the aggregate and remain after the water has evaporated. When a cutback asphalt is used, the hydrocarbon solvent evaporates at different rates depending on the volatility of the solvent. Regardless of the solvent volatility, what remains behind is a paving material where the asphalt component gradually hardens or stiffens over time as the solvent is removed. The binder can alternatively be foamed and mixed with the aggregate to enhance the coating efficacy. While less expensive than hot mixes, cold mixes usually are poorer quality than the hot mixes, and may have poorer binder coating, resulting in less cohesive compaction and durability. Additionally, cutback asphalt mixes have greater environmental impact due to the use of volatile hydrocarbon solvents. Some emulsions also utilize hydrocarbon solvents in addition to water to produce materials suitable for specific applications.
Recently, in an attempt to combine the advantages of hot-mix and cold-mix processes, warm-mix processes have been reported. In one example of a warm-mix process, both “soft” (a component with a lower viscosity than a “hard” component at a given temperature) and “hard” (a component with a higher viscosity than a “soft,” component at a given temperature) components of a bituminous binder are used. The soft component is melted and mixed with aggregate at about 110-265° F., depending on the particular soft component. The heated hard component is then mixed with warm water so as to produce a foam which is mixed with the heated soft component/aggregate mix to achieve a final, coated, paving material. Although a warm-mix paving material can be paved at lower temperatures than hot-mix materials, it requires a more extensive and complex process to produce the warm mix compared to a hot mix.
However the aggregate is coated, if the binder does not adhere well to the aggregate, the binder can separate, or “strip” from the aggregate, causing the material not to remain well-compacted and thereby reducing the overall strength of the pavement. To aid adherence of the binder to the aggregate, the aggregate or more generally the asphalt binder may be treated with an antistripping compound or material (e.g., surfactants) to substantially reduce binder stripping by acting as a binding agent between the aggregate granules and bituminous binder.
Regardless of the aggregate/binder mixing process used, it is essential that the coated paving material not harden together during transport or while in the paving machine or not lack the ability to be compacted to the proper density. Inadequately coated aggregate material, though easy to mix and handle, can cause the paved material not to remain compacted, not to properly support traffic, or not to wear and weather well.